Easy-to-clean cooking surface

ABSTRACT

The invention concerns a food cooking surface for a kitchen utensil or a cooking appliance, characterized in that said cooking surface is made of a metal alloy of zirconium and of at least another metal, and whereof the zirconium content is not less than 75%. In accordance with one embodiment of the invention, the alloy contains less than 10% of elements added to zirconium.

The present invention relates to the field of articles intended for thepreparation and the cooking of food and more particularly the cookingsurface of these articles in contact with food to be treated.

For many years, significant efforts have been developed in order tofacilitate the daily preparation of meals. Among the notable progress,coatings based on fluorocarbonated polymers as adhesive coating inkitchen utensils quickly developed since the end of the 1950's. Suchcoatings is universally known since the process presented in the patentFR 1120749 allowed a reliable attachment of such coatings on variousmetals, such as aluminum.

However, such coatings remain fragile. Thus, clever ways were developedin order to mechanically reinforce the layer on its support. Manyimprovement patents describe methods and means allowing the scratchresistance of such coatings to be increased, by acting on the coatingand/or the substrate. Despite everything, such coatings remain sensitiveto the repeated use of sharpened or pointed metallic materials, such asknives or forks.

In parallel, developments were carried out on mechanically resistantsurfaces on which attempts were made to improve the ease of cleaning.Metal deposition, such chromium plating on stainless steel,quasi-crystals, or nonmetallics (silicates, . . . ) thus appeared.

Quasi-crystals are a phase or metal compound presenting, at thecrystallographic level, symmetries of axial rotation of the order of 5,8, 10 or 12, like the isocahedral and decagonal phases. Such coatingsare in particular described in the patent EP 0 356 287 and have thequalities of scratch resistance, even of anti-adherence in certaincases.

The present invention aims at remedying the above mentioneddisadvantages of the prior art, by offering a cooking surface withimproved characteristics of ease of cleaning, of corrosion resistance,while having a good mechanical resistance.

The present invention is achieved by a food cooking surface for akitchen utensil or cooking apparatus, characterized in that this cookingsurface is a metal alloy of zirconium and at least one other metal, andthe zirconium content of which is at least 75%.

Metal alloys based on zirconium, such as zircaloys, are generally knownfor their exceptional resistance to corrosion, and their good mechanicalresistance to continuous neutronic exposure, while remaining transparentto thermal neutrons. They are primarily used in the nuclear industry asan envelope for uranium oxide fuel bars. Such alloys comprise primarilyzirconium with some alloy elements such as tin, iron, chromium andnickel.

Surprisingly, it was noted, during tests, that alloys of a plurality ofmetals and containing a majority of zirconium, also presented propertiesof ease of cleaning when such surfaces were used as a cooking surfaceand that foodstuffs remained attached to the surface, for example aftera calcination of the products that were cooked. This ease of cleaningcan be expressed by the possibility of easily removing elementscarbonized on the cooking surface.

Advantageously, the alloy contains less than 10% of elements in additionto zirconium.

Such materials, of which the alloys termed zircaloys form part, are moreeasily obtaining because the crystallization conditions are lessdisturbed by alloy elements in small amounts.

According to a first mode of implementing the invention, the foodcooking surface for a kitchen utensil or a cooking appliance is obtainedby depositing a suitable thickness of metallic material on a substrate.This deposition can be carried out by one or the other of the followingprocesses: thermal projection of a powder of an adequate granulometry,deposition by electrophoresis of a micro or submicronic powder, cathodesputtering of a massive target. In this last case the target can beobtained by assembly on a copper substrate of one or more sheets ormaterial plates having the desired composition, the aforementionedsheets or plates being obtained either by powder sintering or thermalprojection of powder, or resulting from casting. Generally, all thetechniques of physical vapor deposition can be used. Other techniques,such as hot compaction or electrolytic deposition also can be used.

This implementation has the advantage of using a small amount ofmaterial and of being able to regulate a low thickness of material onthe substrate in order to produce the cooking surface.

All these techniques make it possible, in addition, to obtain depositshaving strong cohesion with the substrate on which they are deposited.The risks of separation of the deposit during use are thus minimized.

According to a second mode of implementation of the invention, the foodcooking surface for a kitchen utensil or a cooking appliance is obtainedby assembly of a crystalline metal sheet having the desired compositionon a substrate. This implementation has the advantage of approaching theknown implementations of assembly of metals, which makes it possible tobe able to adapt known techniques without significant specificdevelopment.

The zirconium alloy sheets can be obtained by techniques known inmetallurgy, such rolling of an ingot resulting from melting of a mixtureof metals.

The assembly of the sheet on the substrate can be carried out by one ofthe following techniques: colaminating, brazing, hot striking, in a wayknown per se. Plating by explosion also can be considered.

Advantageously, the sheet and the substrate undergo, after assembly, astage of working by stamping. The substrate can be composed of one orseveral metal sheet(s) of the following materials: aluminum, stainlesssteel, cast iron, steel, copper.

Other advantages resulting from the tests will appear from reading thedescription which will follow, in relation to an illustrative example ofthe present invention given as a nonlimiting example.

The example of realization of the invention relates to a deposition byPVD, of an alloy called zircaloy 2 (1.5% of tin, 0.14% of iron, 0.10% ofchromium and 0.05% of nickel) on stainless steel. One face of thisdeposit underwent an extensive polishing, close to optical polishing,before the performance of tests, in order to make it comparable withother cooking surfaces so that the tests for evaluation of the ease ofcleaning such a surface, in a domestic cooking use, can be compared.

The system for evaluation of the ease of cleaning makes it possible toquantify the capabilities of a cooking surface to return to its originalaspect after use. This evaluation system comprises the following steps:

-   -   the surface is locally covered with a food mixture of known        composition,    -   this mixture is carbonized in an oven under defined conditions,        for example 210° C. during 20 minutes,    -   after cooling, surface is put to soak during a controlled time        in a mixture of water and of detergent,    -   an abrasive pad is then applied under a defined constraint using        an abrading apparatus (plynometer) on the soiled surface in a        back and forth movement during a given number of cycles,    -   the percentage of correctly cleaned surface is noted and        characterizes the ease of cleaning of the cooking surface.

The tests carried out on various types of surface thus make it possibleto comparatively evaluate the quality of surfaces as to their ease ofcleaning.

Of course, the tests are carried out by respecting the same parametersfor each step of the evaluation system: same food mixture, same surfaceof application the food mixture, same carbonization temperature, . . .

The following comparative table shows the results obtained on threedifferent cooking surfaces, namely polished stainless steel,quasi-crystals, and the zircaloy 2 alloy deposited on stainless steelsuch as previously described, after polishing, in a severe test with afood composition based on milk and rice considered to be difficult toclean once carbonized. Such a test thus makes it possible to highlightwell the differences between the cleaning quality of the surfaces.polished polished Zircaloy 2 stainless Quasi- on stainless steelcrystals steel Quantity of 30% 40% 80% carbonized residue removed

The table shows without ambiguity the very interesting results obtainedwith the alloy zircaloy 2 deposited on stainless steel. Other tests ledon an aluminum base show similar results.

It is to be noted that the number of abrasion cycles on the plynometerwas fixed at 15. This small number of cycles highlights well the qualityof ease of cleaning of the surface according to the invention sincethere remains no more than 20% of the surface soiled after 15 back andforth passes of the abrasive pad.

Repetitive tests after complete cleaning of the surface show that theease of cleaning of the alloy presented is not altered.

Advantageously, during the deposition process, nitriding of the layer iscarried out by the addition of nitrogen. Such a nitriding also can beenvisioned during a development of the different layer, by anappropriate heat treatment following development of said layer. Such anitriding makes it possible to increase the hardness of the layer, whichconfers on the cooking surface a better abrasion resistance.

Other techniques of heat treatment can be employed to increase thehardness of the alloy. For the zircaloy alloys, one can use the waterquenching/tempering from the field b. One can also use solid phasetempering by laser or equivalent.

When the implementation of the invention implies the use of a substrate,the latter is then composed of one or more metal sheet(s) of thefollowing materials: aluminum, stainless steel, cast iron, steel,copper. However, the present invention is not limited to the realizationof a layer small thickness of a crystalline metal compound such aspreviously described, deposited or assembled on a thick substrate, butalso aims at the realization of massive material, with or without asubstrate, the latter, when it is present, not having a role ofmechanical support for the layer, but assuring another function, such asthe thermal distribution of heat for a utensil placed on a heat source(frying pan, sauce pans . . . ).

1. Food cooking surface for kitchen utensil or cooking appliance,characterized in that this cooking surface is a metal alloy of zirconiumand at least one other metal, and the zirconium content of which is atleast 75%.
 2. Food cooking surface for kitchen utensil or cookingappliance according to claim 1, characterized in that the alloy containsless than 10% of elements in addition to the zirconium.
 3. Food cookingsurface for kitchen utensil or cooking appliance according to claim 2,characterized in that it is obtained by the deposit of a selectedthickness of metallic material on a substrate.
 4. Food cooking surfacefor kitchen utensil or cooking appliance according to claim 3,characterized in that the deposit is carried out by cathode sputteringof a massive target.
 5. Food cooking surface for kitchen utensil orcooking appliance according to claim 4, characterized in that the targetis obtained by assembly on a copper substrate of one or more sheets orplates of material having the desired composition, said sheets or platesbeing obtained either by powder sintering or thermal powder projection,or resulting from casting.
 6. Food cooking surface for kitchen utensilor cooking appliance according to claim 2, characterized in that it isobtained by assembly of a crystalline metal sheet on a substrate. 7.Food cooking surface for kitchen utensil or cooking appliance accordingto claim 6, characterized in that the sheet is obtained by rolling of aningot resulting from melting of a mixture of metals.
 8. Food cookingsurface for kitchen utensil or cooking appliance according to claim 7,characterized in that the assembly is carried out by one of thefollowing techniques: colaminating, brazing, hot striking.
 9. Foodcooking surface for kitchen utensil or cooking appliance according toclaim 8, characterized in that the sheet and the substrate undergo,after assembly, a step of working by stamping.
 10. Food cooking surfacefor kitchen utensil or cooking appliance according to claim 9,characterized in that the substrate is composed of one or more metalsheet(s) of the following materials: aluminum, stainless steel, castiron, steel, copper.
 11. Food cooking surface for kitchen utensil orcooking appliance according to claim 8, characterized in that thesubstrate is composed of one or more metal sheet(s) of the followingmaterials: aluminum, stainless steel, cast iron, steel, copper.
 12. Foodcooking surface for kitchen utensil or cooking appliance according toclaim 8, characterized in that the sheet and the substrate undergo,after assembly, a step of working by stamping.
 13. Food cooking surfacefor kitchen utensil or cooking appliance according to claim 7,characterized in that the sheet and the substrate undergo, afterassembly, a step of working by stamping.
 14. Food cooking surface forkitchen utensil or cooking appliance according to claim 7, characterizedin that the substrate is composed of one or more metal sheet(s) of thefollowing materials: aluminum, stainless steel, cast iron, steel,copper.
 15. Food cooking surface for kitchen utensil or cookingappliance according to claim 6, characterized in that the assembly iscarried out by one of the following techniques: colaminating, brazing,hot striking.
 16. Food cooking surface for kitchen utensil or cookingappliance according to claim 6, characterized in that the sheet and thesubstrate undergo, after assembly, a step of working by stamping. 17.Food cooking surface for kitchen utensil or cooking appliance accordingto claim 6, characterized in that the substrate is composed of one ormore metal sheet(s) of the following materials: aluminum, stainlesssteel, cast iron, steel, copper.
 18. Food cooking surface for kitchenutensil or cooking appliance according to claim 5, characterized in thatthe substrate is composed of one or more metal sheet(s) of the followingmaterials: aluminum, stainless steel, cast iron, steel, copper.
 19. Foodcooking surface for kitchen utensil or cooking appliance according toclaim 4, characterized in that the substrate is composed of one or moremetal sheet(s) of the following materials: aluminum, stainless steel,cast iron, steel, copper.
 20. Food cooking surface for kitchen utensilor cooking appliance according to claim 2, characterized in that thesubstrate is composed of one or more metal sheet(s) of the followingmaterials: aluminum, stainless steel, cast iron, steel, copper.
 21. Foodcooking surface for kitchen utensil or cooking appliance according toclaim 1, characterized in that it is obtained by the deposit of aselected thickness of metallic material on a substrate.
 22. Food cookingsurface for kitchen utensil or cooking appliance according to claim 1,characterized in that it is obtained by assembly of a crystalline metalsheet on a substrate.
 23. A kitchen utensil or cooking appliance havinga food contacting surface constituted by the surface defined in claim 1.